On-off thermal switch for a cryopump



Aug. 25, 1970 B. s. DENHOY 3,5 5, 29

ON-OFF THERMAL SWITCH FOR A CRYOPUMP Filed Feb. 6, 1969 INVENTOR. BALWANT S. DENHOY ATTORNEY United States Patent O 3,525,229 ()N-OFF THERMAL SWITCH FOR A CRYOPUMP Balwant S. Denlioy, Brentwood, Calif., assignor to the United States of America as represented by the United States Atomic Energy Commission Filed Feb. 6, 1969, Ser. No. 797,080 Int. Cl. B01d 5/00 US. Cl. 62-555 Claims ABSTRACT OF THE DISCLOSURE An on-oif thermal switch for thermally isolating a cryogenic cooling means from a surface being cooled is incorporated into a cryopump thereby facilitating an increased pumping rate and efiiciency for the pump as well as more economical and less hazardous operating procedure. The inventive switch comprises an insulating space between the cryogenic cooling means and the cooled surface. A gas is introduced into the insulating space to establish thermal contact between the cooling means and the cooled surface. The gas is evacuated to thermally isolate the cooling means.

BACKGROUND OF THE INVENTION The invention described herein was made in the course of, or under, Contract No. W-7405-ENG-48, with the United States Atomic Energy Commission.

This invention relates to an on-off thermal switch for a cryogenic cooling means. More particularly, the invention relates to an on-off thermal switch for a cryopump.

One common technique for obtaining ultra vacua is a cryopump in which a liquified gas such as air, nitrogen or helium cools a pumping surface exposed to gas molecules within a vacuum region. Depending upon the temperature of the pumping surface, certain gas molecules condense upon the surface as solids with extremely low partial pressures whereupon they are removed from the free space in the vacuum region. The pumping capacity of the pumping surface is not unlimited, however, and the pumping rate decreases as the number of gas molecules condensed upon the the surface increases.

The prior art teaches that a regenerative pumping surface, as exemplified by the cryopump of US. Pat. No. 3,156,406 issued Nov. 10, 1964 to W. A. Lloyd et al., for a High Vacuum Pumping Method and Apparatus, increases the pumping rate and the efiiciency of the pumping surface. Once the pumping rate is reduced sufficiently, the pumping surface is isolated from the main vacuum region and allowed to warm thereby causing the gas molecules to evaporate from the surface. A vacuum pump removes the gas molecules from the isolated region and the pumping surface is then rechilled and returned to the main vacuum region.

Although the regenerative pumping surface solved one problem of the prior art, additional problems were encountered. Each regeneration of the pumping surface requires that the liquified gas be removed from thermal contact with the surface. The prior art teaches that this is accomplished by allowing the liquified gas to evaporate. Thermal contact is reestablished by adding additional liquified gas. This procedure is not only relatively expensive, often impractical and always time consuming, but it also involves the hazard of continual handling of the liquified gas.

SUMMARY OF THE INVENTION The present invention solves the problem of the prior art by providing an on-off thermal switch for thermally switching a cryogenic cooling means on and off, thereby circumventing the necessity of removing and replacing the liquified gas. More specifically, a hermetic insulating space is disposed between the cooling means and the surface to be cooled. When the insulating space is filled with a gas, thermal contact is established between the cooling means and the cooled surface. The cooling means is switched ofl by evacuating the gas from the insulating space, thereby thermally isolating the cooling means from the cooled surface.

The inventive thermal switch is compatible with most prior art cryopumps having a regenerative pumping surface and is incorporable therein by providing additionally a hermetic insulating space disposed between the coolant vessel for containing the liquified gas and the pumping surface. A gas source for supplying a gas to the insulating space and a vacuum means for evacuating the gas from the space are also required. A cryopump utilizing the inventive thermal switch has an increased pumping rate and efiiciency. In addition, the inventive switch permits more economical operation of the cryopump in that the time consuming procedure of removing and replacing the liquified gas is eliminated and there is no wasting of the relatively expensive gas. Finally, the hazards involved in the operation of the cryopump are reduced since handling of the liquified gas is kept to a minimum.

BRIEF DESCRIPTION OF THE DRAWING The single figure is a diametrical section of an embodiment of a cryopump utilizing an on-ofi thermal switch of the present invention.

DESCRIPTION OF THE INVENTION Referring to the figure, cryopump 11 is comprised of a right-circular cylindrical outer housing 12 communicatively connected to a vacuum region 13 to be evacuated by the cryopump 11 and is isolated therefrom by valve 14. A double-walled, right-circular cylindrical radiation shield 15 is coaxially supported within the outer housing 12, thereby defining a plenum region 16 between the radiation shield and the outer housing 12. The space between the walls of the radiation shield 15 is filled with liquid nitrogen 17. A right-circular cylindrical coolant vessel 18 for containing a liquified gas is coaxially supported within the radiation shield 15. A liquified gas 19 such as nitrogen or helium is supplied to the coolant vessel 18 through a suitable filler tube 20 which extends through the radiation shield 15 and the outer housing 12, and serves to support the radiation shield 15 and the coolant vessel 18 within the outer housing 12.

An upper portion of the coolant vessel 18 is enclosed within an insulation housing 21 being attached to the cylindrical wall of the vessel 18 and to the filler tube 20, thereby defining a hermetic insulating volume 22 therebetween. The insulating volume 22 is evacuated. The remaining lower portion of the coolant vessel 18 is enclosed within an inner housing 23, being attached to the upper housing 21, thereby defining a thermal switching volume 24 therebetween. The inner housing 23 is constructed of a highly conductive material, for example, copper, to provide a pumping surface 25 for the cryopump 11. The radiation shield 15 and the insulation housing 21 and inner housing 23 define a plenum region 26 therebetween. The plenum region 16 and the plenum region 26 are communicatively connected by passages 27 through the lower end of the double-walled inner housing 15.

A gas source 28 containing a gas such as helium, and a vacuum pump 29 are communicatively connected to the thermal switching volume 24 by means of piping 30, extending inside the filler tube 20 and through the bottom of the coolant vessel 18, and valves 31 and 32 serve to isolate the gas source 28 and the vacuum pump 29, re-

spectively, from the switching volume 24. Vacuum pump 33 is communicatively connected to plenum region 16, and to plenum region 26 through passages 27, and is isolated therefrom by valve 34. When valves 31 and 32 are closed, the thermal switching volume 24 is a hermetic volume. Likewise, when valves 14 and 34 are closed, plenum region 16, and plenum region 26 communicatively connected therewith, are a single hermetic volume.

The double-walled radiation shield 15 filled with liquid nitrogen 17 prevents external radiant energy from impinging upon the voolant vessel 18. Aditional thermal protection is afforded the coolant vessel 18 by the hermetic insulating volume 22. Preferentially, the passages 27 through the double-walled inner housing 15 are located so as to afford maximum shielding for the pumping surface 25 from radiant energy within the vacuum region 13.

The cryopump 11 operates as follows: with valve 14 open and all other valves closed, the vacuum region 13 and the plenum regions 16 and 26 are partially evacuated by means of a vacuum pump, not shown, such as a diffusion pump. The liquified gas 19 is then supplied to the coolant vessel 18 through filler tube 20 and the thermal switching volume 24 is filled with a gas from the gas source 28. The gas in the thermal switching volume 24 establishes thermal contact between the coolant vessel 18 and the inner housing 23 thereby cooling the pumping surface 25. Condensable gases still present in the partially evacuated plenum region 26 begin to condense as a solid upon the pumping surface 25,. When the pumping surface has reached its capacity to condense the gases, the plenum regions 16 and 26 are isolated from the vacuum region 13 by closing valve 14, and the thermal switching volume 24 is evacuated by vacuum pump 29 thereby thermally isolating the pumping surface 25 from the coolant vessel 18. As the pumping surface begins to warm, the condensed gases are evolved from the pumping surface 25 into the plenum regions 16 and 26. Vacuum pump 33 then evacuates the plenum regions 16 and 26 thereby removing the evolved gases so that they cannot return to the vacuum region 13 when valve 14 is opened. The operation described above is repeated until the desired vacuum is reached in the vacuum region 13. When the regenerative cycling of the cryopump 11 is completed, valve 14 is closed to fully isolate the cryopump 11 from the vacuum region 13.

The inventive on-otf thermal switch increases the pumping rate and the elficiency of the cryopump described above by facilitating the regenerative cycling process and, in addition, permits more economical and less hazardous operation of the pump. The time consuming procedure of removing and replacing the liquified gas in the coolant vessel, as the prior art teaches, is eliminated and the amount of liquified gas consumed during the operation of the pump is substantially decreased, thereby minimizing handling of the gas.

Although the present invention is described above in the context of a cryopump, it is to be understood that the inventive on-off thermal switch can be utilized with any cryogenic cooling means. It will be apparent to those skilled in the art that modifications and changes can be made to the inventive switch without departing from my invention in the broader aspects; and I therefore do not intend to limit the scope of my invention except as defined in the appended claims.

I claim:

1. An on-off thermal switch for a cryogenic cooling means having a coolant vessel for containing a liquified gas comprising: an inner housing to be cooled, the coolant vessel being at least partially disposed within the inner housing in spaced relation thereto so as to define a thermal switching volume between the coolant vessel and the inner housing, the switching volume being a hermetic volume, a suitable filler tube for supplying the liquified gas to the coolant vessel, a means for introducing a gas into the switching volume, and a means for evacuating the gas from the switching volume whereby the introduction of the gas into the switching volume establishes thermal contact between the coolant vessel and the inner housing and the evacuation of the gas from the switching volume thermally isolates the coolant vessel from the inner housing.

2. The on-ofi thermal switch defined in claim 1 in combination with a cryopump comprising an outer housing including a valve means for communicatively connecting the outer housing to a vacuum region which is to be evacuated to a high vacuum, the inner housing, including the coolant vessel, being disposed within the outer housing in space relation thereto so as to define a plenum region between at least the inner housing and the outer housing, the plenum region being a hermetic volume when the valve means is closed, the inner housing having an outer surface functioning as a pumping surface of the cryopump, and a means for evacuating the plenum region when the valve means is closed.

3. The combination of claim 2 in further combination with a radiation shield disposed within the outer housing in spaced relation thereto so as to divide the plenum region into first and second plenum areas, the first plenum area being between the radiation shield and the outer housing, the radiation shield enclosing the inner housing and the coolant vessel thereby defining the second plenum area between the radiation shield and the inner housing, the second plenum area being in fluid communication with the first plenum through passages in the radiation shield.

4. The combination of claim 3 further defined in that the radiation shield is a double-walled radiation shield having a space between the walls, the space being filled with a liquid gas.

5. The combination of claim 4 further defined in that the liquid gas is liquid nitrogen.

6. The combination of claim 4 in further combination with an insulation housing, the insulation housing enclosing a portion of the coolant vessel not enclosed by the inner housing, thereby defining a hermetic insulating volume between the coolant vessel and the insulation housing.

7. The combination of claim 6 further defined in that the inner housing is constructed of a highly conductive material.

8. The combination of claim 7 further defined in that the highly conductive material is copper.

9. The on-olf thermal switch of claim 1 further defined in that the liquified gas is selected from the group consisting of nitrogen and helium.

10.. The on-oif thermal switch of claim 1 further defined in that the gas introduced into the switching volume is composed essentially of liquid helium.

References Cited UNITED STATES PATENTS 2,985,356 5/1961 Beecher 62-555 3,256,706 6/1966 Hansey 62-555 3,270,802 9/1966 Lindberg 62-45 3,483,709 12/1969 Barcker 62-45 WILLIAM J. WYE, Primary Examiner U.S. Cl. X.R. 

